1. Field of the Invention
The present invention relates to a method for discharging a flash lamp and, more particularly, to such a method wherein the flash lamp discharge is triggered by radiant energy.
2. Description Relative to the Prior Art
A standard flash lamp cmprises a pair of electrodes sealed in a glass or quartz envelope which is filled with an inert gas such as xenon, krypton, neon, etc. As the lamp is flashed, ions and electrons bombard the electrodes and cause electrode material to be scattered and deposited on the inner surface of the lamp envelope. Blackening of the envelope occurs which reduces the light output and increases the electrical impedance of the lamp. The flash lamp eventually fires irregularly and, finally, fails to fire at all. This cause of flash lamp failure, which is one of the more common and more serious causes, is commonly termed "electrode sputtering".
Electrode sputtering is a major problem in flash lamps that are operated with series-injection triggering, especially when the flash lamp is repetitively pulsed. This form of triggering employs a high voltage pulse applied directly to the lamp electrodes. The high voltage pulse ionizes a conduction path between the lamp electrodes, thus lowering the impedance of the flash lamp. A lower voltage but higher energy pulse immediately follows the trigger pulse and produces the lamp flash. As might be expected, arcing a high voltage pulse (commonly on the order of 20-30 kv) between the lamp electrodes places severe stress on the electrodes and often leads to early lamp failure.
To reduce stress on the electrodes, the flash lamp may be triggered externally by means of a trigger wire placed in close proximity to the lamp envelope (it may, for example, be wrapped around the envelope). A high voltage pulse is applied to the trigger wire causing some ionization to take place within the lamp. As before, a lower voltage but higher energy pulse is then applied to the lamp electrodes to produce the flash output.
External triggering, however, is not without problems. Because a high voltage pulse (on the order of 30 kv) is applied to an external trigger wire, spurious arcing can occur between the trigger wire and other components of the flash lamp apparatus such as the lamp electrodes, grounded metal parts, mounting hardware, etc. Also, the high voltage can cause discoloration of the lamp envelope thereby reducing the useful light output of the flash lamp. External triggering is also less reliable than series-injection triggering, particularly for lower trigger voltages. Lastly, the external trigger wire is physically cumbersome and requires that special provision be made for its placement in the lamp housing.
More recently, flash lamps have been operated in what has been termed a "simmer" or "keep alive" mode of operation which requires the addition of a simmer power supply connected in parallel to the flash lamp. The flash lamp is triggered by conventional means (i.e., a high voltage trigger pulse) to allow the simmer power supply to establish a low current dc arc of about 20-100 ma between the flash lamp electrodes. The simmer arc is of insufficient current to provide useful light output, its purpose being to maintain a low impedance, ionized conduction path between the lamp electrodes. To flash the lamp, a switching circuit applies the flash voltage to the lamp electrodes. Because the ionized conduction path provides a low impedance path for the flash voltage, the lamp fires without additional triggering. An advantage of the simmer mode of operation is that the simmer arc is not extinguished upon firing of the flash lamp. The flash lamp can therefore be operated in a pulsed mode by repetitively applying the flash voltage. Since the high voltage trigger pulse is employed only once to initiate the simmer arc, lamp life is significantly increased.
The simmer mode of operation suffers certain disadvantages, however, in that maintenance of a simmer arc in the flash lamp consumes additional power and generates more heat. Additional cooling apparatus is therefore required to maintain the flash lamp at the proper temperature. Under actual operation conditions, the simmer arc may be accidentally extinguished for various reasons, such as excessive heat build-up in the flash lamp. When the simmer arc is extinguished it is necessary to use the high voltage trigger pulse to re-start the arc. As, previously discussed, the more times a high voltage trigger pulse is applied to a flash lamp, the greater the chances are of premature lamp failure.